A medical device, such as an orthopedic or prosthetic joint, has a protective coating bonded to the substrate material of the device. Suitable substrate materials may include pure metals and metal alloys, ceramics, polymers and composites of the above. The protective coating includes a thin layer of tetrahedral bonded Carbon (ta-C). The coating also optionally includes an interface layer to facilitate the initial bonding and retention of the ta-C layer, such as by acting as an adhesion promoter, fluid barrier layer, or coefficient of thermal expansion mismatch reducing layer, or combination thereof. The interface layer may include various tightly adherent metals and metal nitrides, such as Cr, Ti, Nb, Ta and carbides, nitrides and carbonitrides thereof. The ta-C layer has a concentration of sp3 bonded carbon which varies through its thickness. Many concentration profiles of sp3 carbon bonds through the thickness are possible. The ta-C layer may also be doped with various materials, either through its thickness, or at either an inner or an outer interface, or both. The doping may include any suitable dopant, including various pure metals or metal alloys, bone growth substances, and dopants which alter the tribology of the ta-C layer with respect to the fluids in which it is in contact, such as dopants comprising F and N, or combinations of these materials.